These doors and windows normally comprise:                a fixed frame;        at least one mobile frame, or sash, that slides horizontally relative to the fixed frame between an open position and a closed position;        at least one sliding unit, or carriage unit, associated with the bottom sash rail and resting on a horizontal base track to enable the sash to slide in both directions to and from the open and closed positions;        a control handle mounted on the sash and enabling the sash firstly to be lifted so as to move the sides of the track away from the weather-strips at the bottom of the sash to allow the latter to slide to and from the open and closed positions, and secondly to be lowered when it reaches the closed position so that the weather-strips seal the door or window substantially hermetically;        locking/unlocking means acting between the vertical member of the sash and the fixed frame to lock/unlock the sash in the closed position under the action of the handle.        
This text focuses on the carriage unit which, in most of the constructional solutions currently available on the market, comprises a pair of carriages (main and secondary) so as to support the sliding sash in a balanced manner.
Usually, as is known for example from prior patent publications DE 7816563U, EP 1.298.271, EP 1.298.272, EP 1.437.471, each carriage comprises: a box-shaped frame that supports at its ends a pair of wheels designed to run on the track; and a connecting member interposed between the box-shaped frame and the bottom of the sliding sash to enable the sash to be lifted and lowered relative to the box-shaped frame.
In particular, in a prior art structure, the connecting member is, on one side, connected to the bottom of the sash and, on the other, connected to the box-shaped frame by a coupling between a pin and an inclined slot (applied to, and formed in, the box-shaped frame and connecting member, or vice versa, depending on the solution adopted).
The movement of the connecting member (and of the sash) relative to the box-shaped frame of the carriage is imparted by the above mentioned control handle through a drive system which changes the relative position between the pin and the inclined slot.
In the solutions known up to now, the movement for lifting the sash requires a considerable effort of the user, especially if the sash handle, for aesthetic reasons, has a relatively short control lever.